Changes in afferent input can have profound effects on the morphology and metabolism of neurons in the central nervous system (CNS), even resulting in the death of the postsynaptic cell. The specific cellular events which precede neuronal death in the CNS are not clearly understood. Chick brainstem auditory neurons provide a model system in which to investigate these cellular events. Second-order auditory neurons in n. magnocellularis (NM) receive their only excitatory afferent input from the ipsilateral cochlea via the eighth nerve. Unilateral cochlea removal in hatchlings produces rapid changes in NM neuronal number, size, metabolism, and ultrastructure. Few such changes are seen after cochlea removal in adult animals. We have shown that an increase in oxidative enzyme activity and proliferation of mitochondria occurs in most deafferented NM neurons within hours of cochlea removal in young birds. Mitochondria in neurons likely to die after deafferentation show abnormal morphology, suggesting that they may be functionally deficient. Blockade of mitochondrial protein synthesis with chloramphenicol increases cell death in NM. These observations suggest that mitochondrial proliferation in NM may be involved in neuronal survival following deafferentation. In the first set of experiments we will determine the time course and magnitude of changes in mitochondria in deafferented NM neurons. We will assess oxidative enzyme activity, mitochondrial "coupling state", and levels of high energy intermediates. We then will use blockade of mitochondrial proliferation with chloramphenicol to examine these indices in animals with exaggerated cell death. We will vary the period after cochlea removal in which chloramphenicol is given to determine the "critical period" during which mitochondrial proliferation can affect neuronal survival. We will increase resting levels of mitochondria with thyroid hormone to see whether less cell death is observed after deafferentation. Finally, we will examine quantitatively the relationship between afferent input and the severity of neuronal cell death. The biological basis for the age difference in susceptibility to removal of afferent input is unknown. We will examine morphological and metabolic markers identified in hatchlings to determine whether they might differ in adults. If any differences in mitochondrial morphology or function are observed, we will block mitochondrial protein synthesis in adults to see whether neuronal cell death in NM can be induced.